Funding is requested to purchase a low dose rate, dual source, laboratory irradiator (Gammacell 40) to meet the needs of transplantation and cancer immunology programs supported by the National Institutes of Health in the Departments of Pediatrics, Microbiology and Immunology, Medicine, and Ophthalmology in the School of Medicine and in the Rosensteil School of Oceanography. The programs are primarily concerned with understanding the basic mechanisms of the immune response to foreign tissues. These programs include studies of bone marrow transplantation, graft-vs-host disease, T and B cell differentiation, the mechanism of graft rejection, the maternal immune responses to antigens of the fetus, immune responses to viruses and other antigens in the eye, and the mechanism of resistance to tumors in fishes. The need to purchase the Gammacell 40 irradiator exists because the animals used in these studies are housed in an isolated vivarium facility that is approximately two miles from the main Medical School campus. Transport of animals from this closed facility to the main campus for irradiation induces significant stress on the animals and puts the animals at risk of significant infection because transportation necessarily violates the protocols established to maintain the vivarium as a Specific Pathogen Free facility. Acquisition of a Gammacell 40 irradiator that will be housed in this isolated facility will increase productivity both by decreasing transportation time and by decreasing the risk of intercurrent infections which may significantly alter experimental results. The investigators that will primarily use the Gammacell 40 represent only a small fraction of the total number of individuals that use the existing radiation source at the Medical School. Moving the existing source to the isolated vivarium facility is not feasible because of cost and safety concerns and because a large number of other investigators require the use of the existing unit. The Gammacell 40 will be available to outside investigators for uses that do not violate the SPF barriers of the colony.